Imaging devices form images on image receiving members that include paper and other print media. Different imaging or printing techniques, which include laser printing, inkjet printing, offset printing, dye-sublimation printing, thermal printing, and the like, may be used to produce printed documents. In particular, inkjet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals. The size of the ink drops and the timing of the ejection of the ink drops are affected by the frequency and amplitude of the firing signals.
Throughout the life cycle of imaging devices, the image generating ability of the device requires evaluation and, if the images contain detectable defects, correction. Various defects in the image generating process affect ink image quality. In an inkjet printing system, one such defect occurs when an individual inkjet becomes inoperable as either a “weak” or “missing” inkjet. A weak inkjet intermittently ejects ink drops or ejects ink drops having a mass that is different than expected for the firing signal used to operate the actuator for the inkjet. A missing inkjet fails to eject ink drops entirely. Inoperable inkjets, including both weak and missing inkjets, negatively impact the quality of printed images.
Some existing printing systems are configured to detect and compensate for weak or missing inkjets. Identifying inoperable inkjets typically requires the printing of reference patterns, which are specially designed and arranged ink lines that are printed on the image receiving member. These reference patterns must be printed separately from the images printed as part of a print job. Consequently, the printing of reference patterns absorbs a portion of the resources to be used for productive printing. Because a printhead often includes hundreds or thousands of individual inkjets, correct identification of a single inoperable inkjet presents challenges. In some imaging devices, an optical sensor is used to generate image data of the reference pattern on an image receiving member and these data are analyzed and correlated to inkjet positions in a printhead to identify a weak or missing inkjet. Errors in the alignment of the photosensors in the optical sensor or in the calibration of the sensor along with distortions that arise from media shifting during operation of the printer affect the accuracy of the analysis of the image data. Consequently, improvements to the identification of weak or missing inkjets in an inkjet printer would be beneficial.